Could the United States be Close to Reaching Herd Immunity Against COVID-19?
Could more than 80% of Americans already have been infected with COVID-19?
COVID-19 has quickly spread across the United States, and virtually no one has been unaffected by the virus or various entities’ response to it. This post explores the difference between reported COVID-19 cases and the Centers for Disease Control and Prevention’s estimate of COVID-19’s disease burden. I am in no way claiming to have any expertise or answers as to what the herd immunity threshold is for COVID-19.
It is impossible for official case counts that rely on positive COVID-19 tests to capture every case of COVID-19. Many people experience an asymptomatic case, and would have no reason to seek a test. Many others may have mild symptoms and choose not to seek a test because they believe they have a cold or flu, or simply don’t want to deal with the hassle of seeking a test.
The CDC also recognizes this, and has developed a statistical model to estimate the total number of COVID-19 cases in the United States, including those that never appear on a positive test result. The paper explaining this model can be found at this link: Estimated incidence of COVID-19 illness and hospitalization — United States, February–September, 2020 | Clinical Infectious Diseases | Oxford Academic (oup.com). The paper estimates there were 52.9 million COVID-19 infections in the United States between February 27, 2020 and September 30, 2020. This would mean 16% of the U.S. population was infected between the end of February and the end of September 2020. (United States Population (2021) — Worldometer (worldometers.info)
The CDC appears to have recently updated this estimate, and its “Estimated Disease Burden of COVID-19” page now estimates that 91 million Americans were infected between February and September 2020. (Estimated Disease Burden of COVID-19 | CDC) This estimate suggests that 27% of the American population was infected between February and September 2020.
It further suggests that the actual number of COVID-19 cases was approximately 12.69 times greater than the number reported during this time period. According to data compiled by The Atlantic’s COVID Tracking Project, states reported 7,173,318 COVID-19 cases between February and September 2020. (US Historical Data | The COVID Tracking Project)
If we assume that actual COVID-19 cases have been approximately 12.69 times greater than the reported number of cases, then 267,293,653 Americans have already had COVID-19. This number is calculated by multiplying the total number of cases in The Atlantic’s COVID Tracking Project as of January 6, 2021 (21,070,136) by 12.69. (US Historical Data | The COVID Tracking Project) If the CDC’s estimate of the difference between reported and actual COVID-19 cases is accurate and consistent throughout the pandemic, then 80.75% of the American population has been infected with COVID-19. This is substantially greater than 6.37% of the population infected based on reported cases. Applying the CDC’s own estimates, the United States could be much closer to herd immunity than reported cases suggest.
What is herd immunity?
According to the Mayo Clinic, “[h]erd immunity occurs when a large portion of a community (the herd) becomes immune to a disease, making the spread of the disease from person to person unlikely. As a result, the whole community becomes protected — not just those who are immune.” (Herd immunity and COVID-19 (coronavirus): What you need to know — Mayo Clinic)
What is the herd immunity threshold for COVID-19?
I want to reiterate that I am in not claiming to have any expertise or answers as to what the herd immunity threshold is for COVID-19. According to Johns Hopkins, the herd immunity threshold for most diseases is between 50% and 90% of a population. (What is Herd Immunity and How Can We Achieve It With COVID-19? — COVID-19 — Johns Hopkins Bloomberg School of Public Health (jhsph.edu) More contagious diseases have a higher herd immunity threshold than less contagious diseases. Dr. Gypsyamber D’Souza, an epidemiologist at Johns Hopkins, has estimated that the herd immunity threshold for COVID-19 is 70% of the population. (https://youtu.be/HfRN-UjEeZo) According to The New York Times, Dr. Anthony Fauci’s most recent herd immunity threshold is close to 90%. This would be a similar threshold as measles. (Covid-19: How Much Herd Immunity is Enough? — The New York Times (nytimes.com))
Again, I do not pretend to have any idea what the herd immunity threshold for COVID-19 is, but it is worth noting that the CDC’s estimates of total case load indicate the United States could be closer to the threshold than the number of reported cases suggest.
